1. Field of the Invention
The present invention relates to a system and method employing algorithms and protocols for optimizing carrier sense multiple access with collision avoidance (CSMA/CA) in wireless communications networks. More particularly, the present invention relates to a system and method for providing enhanced CSMA/CA which improves channel availability and quality of service (QoS) in a wireless communications network, such as an ad-hoc wireless communication network.
2. Description of the Related Art
In 1987, Apple was awarded U.S. Pat. No. 4,661,902, the contents of which is incorporated by reference herein, for a CSMA protocol for wired LANs based on a request to send/clear to send (RTS/CTS) exchange (referred to as “AppleTalk”). The CSMA scheme allows multiple devices to share a common resource (i.e., the same physical cable) for communication. Such a scheme is very effective when the communication takes the form of bursts of packets rather than a constant stream of data, such as time division multiplex (TDM) voice.
In 1991, Proxim was awarded U.S. Pat. No. 5,231,634 entitled “Medium Access Protocol for Wireless LANs”, the entire content of which is incorporated herein by reference, that describes an enhanced carrier sense multiple access with collision avoidance (CSMA/CA) protocol for a wireless local area network (LAN). The protocol specifically addresses the hidden terminal problem that exists in wireless networks, in which not all terminals are aware of each others existence and the transmissions that each other are making. The protocol also includes an attempt to derive a fairness of access so that the terminals closest to each other do not monopolize the radio link.
The ITT HandHeld Multimedia Terminal (HMT) system as described in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference, also uses a form of enhanced CSMA/CA protocol which includes the RTS/CTS as defined by Apple. The HMT technology allows each wireless terminal to function as an independent, intelligent router and repeater. Its ad hoc networking provides for a self-organizing and self-healing network structure. The HMT system is a significant improvement over the protocol described in the Proxim patent in a number of ways. Specifically, the HMT system is designed for use by a self forming/self healing network of highly mobile devices in which a simple RTS/CTS is insufficient as described below. Also, the HMT system introduces the concept of three data channels with a common reservation channel to optimize the use of the radio frequency (RF) spectrum. The Proxim protocol does not do this presumably because of the low mobility of the terminals in a wireless LAN and the probability of a large number of interferers being limited by lack of free space propagation. That is, wireless LANs are typically deployed in buildings where walls create natural barriers to RF propagation. However, in the highly mobile open air environment, this assumption is not reasonable.
The Proxim protocol also uses an RTS/CTS and data packet exchange in a common channel. In accordance with this process, no other terminal can use the radio resource while the exchange is occurring or during the back-off period that they define, which results in very low efficiency of the radio resource. As discussed above, the HMT system employs a mechanism with a single shared reservation channel on which all RTS/CTS communication occurs, and three data channels are used for the actual sending of data. As part of the RTS/CTS exchange, the terminals agree on a data channel to use. Accordingly, while a single pair of terminals is transmitting a data packet on one data channel, other terminals can correspond over the reservation channel to set up a packet exchange on another data channel. The result is that the overall efficiency of the network in terms of packets sent per second is much higher.
Although the HMT system operates well for its intended purpose, the system was designed for a military application where all terminals knew about all others (i.e., an all informed network). However, it would be beneficial to employ these techniques in a network in which each terminal does not necessarily have information about each other terminal. Accordingly, a need exists for an improved CSMA/CA protocol suitable for use with such a network.